As for an electronic medium replacing paper, developments of electronic paper have been recently actively carried out.
The electronic paper has characteristics that the display device thereof is used like paper, and therefore requires properties different from conventional display devices, such as CRT and LCD. For example, required properties thereof are being a reflective display device as well as having high white reflectance and high contrast ratio, being able to display with high definition, giving the display a memory function, being driven at low voltage, being thin and light, and being inexpensive. Among them, as properties associated with a quality of a display, particularly white reflectivity and contrast ratio close to that of paper are highly demanded.
Previously, as for a display device for use as electronic paper, for example, proposed are a system using reflecting liquid crystals, a system using electrophoresis, a system using toner migration, and the like. Among them, a mainstream is an electrophoresis system, and the electrophoresis system has been system, however, it is currently widely used in electronic paper on a market. In this particularly difficult to provide high white reflectance. It has been known that the white reflectance provided by this system is low, i.e., about 40%, whereas the value thereof of paper is 80%, and the value thereof of news paper is 60%. Therefore, to provide high white reflectance is a large task for this system.
As for a promising technology for solving the aforementioned problem and realizing a reflecting display device, there is a system using electrochromic phenomenon. The phenomenon where, as voltage is applied, an oxidation-reduction reaction is reversibly caused depending on the polarity to thereby reversibly change color is called electrochromism. A display device utilizing coloring/discoloring (may also referred to as coloring and discoloring hereinafter) of the electrochromism is an electrochromic display device. Since this electrochromic display device is a reflecting display device, has a memory effect, and can be driven at low voltage, researches and developments of electrochromic devices have been widely conducted from a development of materials and designing of devices, as a promising option for a display device technology for electronic paper. It has been confirmed that white reflectance achieved by this method, i.e. 60%, is substantially the same value to that of paper (PTL 1).
An electrochromic display device is a system that can solve the majority of the aforementioned problems. The electrochromic compound used in a monochrome display element disclosed in PTL 2, which is produced using the electrochromic system, presents a color, which is slightly tinted with yellow, in a discolored state, and high white reflectance, which is one of characteristics of the electrochromic system, has not been achieved.